In a Docker container, gems at a path can't be installed

When running bundle install in a Docker container, only Gemfile entries at a URL can be installed — Gems at a local path can't unless pre-copied, forcing a (possibly-premature) remote commit.

This is expected, because even if the container has been given access to the local file system, bundle is unaware of it.

One solution would be to add a --home-path option to bundle install, which provides the path into a mounted file system equivalent to the non-container Gemfile location, to which Gem paths are relative.

For example, if an entry in the Gemfile in the myapp directory is

gem 'xyz', path: '../xyz'

Then by mapping this myapp parent directory as the root of a file system mapped into the container at path /local

bundle install --home-path=/local/myapp

can now install paths like the above, now relative to the home-path setting.

Absolute paths would require mounting the whole file system. Interpreting ~ home directory path leaders would be helpful.

Start by tracing the bundle install option handling and local path dependency resolution; no source files or tests are named in the issue. Use the Docker and Gemfile examples to define how --home-path should resolve relative and ~ paths, then add coverage showing that mounted local gems install without being pre-copied.